Questions relating to safety

During construction of Wendelstein 7-X IPP was faced with a series of questions relating to radiation protection and safety of the fusion device. Here the most important questions and IPP’s answers to them.

Queries were particularly numerous in the summer of 2012, after BUND M-V e.V. publicly expressed doubt about the safeness of the device. The Social Ministry of the State of Mecklenburg-Vorpommern and the state authority concerned, viz. the State Office for Health and Social Matters (LAGuS), thereupon decided to commission an independent expert to appraise the existing expertise and calculations relating to radiation protection. This was expressly supported by IPP as an important step on the way to applying for approval to put Wendelstein 7-X into operation.

The TÜV Süd expert started inspection in January 2013, his 118-page expertise being published at the end of October 2013. The main result: It was certified that the Wendelstein 7-X research device meets the radiation protection requirements in every respect.

Is Wendelstein 7-X a nuclear installation (“kerntechnische Anlage”)?No. Unlike research reactors or nuclear power plants, the Wendelstein 7-X fusion device is not a nuclear installation. This is of great importance for the permit procedure: Whereas research reactors at universities or research institutes are assessed like commercial nuclear power plants in accordance with the regulations of the German “Atomgesetz” (Atomic Energy Act), Wendelstein 7-X, as a device that can produce “ionising radiation”, is subject to the regulations of the “Strahlenschutzverordnung” (Radiation Protection Ordinance). The same ordinance also applies, for example, to devices in nuclear medicine and to dealing with radioactive substances or ionising radiation in medical practices or hospitals. Particle accelerators operated in research are also subject to the Radiation Protection Ordinance. In all cases testing in accordance with the regulations of the ordinance ensures that the device cannot constitute any risk to personnel or the public.

What type of radiation will Wendelstein 7-X produce after commissioning?In the first few years Wendelstein 7-X will experiment with a plasma consisting of light hydrogen. The X-radiation thereby produced is completely taken up by the walls of the plasma vessel.

Experiments with plasma consisting of heavy hydrogen (deuterium), which are planned for subsequent years, additionally entail release of neutrons. According to the construction permit Wendelstein 7-X is allowed to produce a maximum of 3 x 1019 neutrons p.a. These neutrons are captured by the concrete of the experimentation hall. Outside the hall there is just a radiation load that is well below the natural background radiation. Inside the hall the neutrons could slightly activate the structures present, so that gamma radiation would be emitted there. As cobalt is mainly responsible for this, only special low-cobalt steel is used in the interior of Wendelstein 7-X. After a plasma experiment is terminated, the experimentation hall will be accessible to scientific and technical personnel.

Why are the concrete walls of the Wendelstein hall 1.80 metre thick? The water and boron contents of concrete afford particularly good protection against neutrons. As a hall had to be built anyway for Wendelstein 7-X, it seemed reasonable to make the walls thicker and include appropriate amounts of water and boron. X-rays, neutrons or gamma rays are also present in radiological clinics. Depending on the radiation involved, various materials are chosen for shielding – concrete, but also paraffin or lead.

What is IPP’s view of the doubts about safety, expressed in 2012 with respect to the composition of the concrete?The subject of discussion in the years 2012/2013 was a condition in the construction permit for Wendelstein 7-X which was granted by the Social Ministry of Mecklenburg-Vorpommern on 18 December 1997: In accordance with the construction permit, an expert was appointed for the building phase to ensure that the concrete chosen for the experimentation hall is of the quality prescribed, viz. with a sufficiently high boron (>1000 ppm) and water (>120 l/m3) content. The boron material, which acts as a neutron absorber, is not completely uniformly distributed in the concrete. The relatively small samples therefore exhibited slightly different values. The average value of the 48 sample measurements was 1030 ppm, i.e. above the value prescribed. But 8 of the 48 values were below 1000 ppm, as statistically expected. Accordingly, IPP in 1997 got Gesellschaft für Anlagen- und Reaktorsicherheit mbH (GRS) to check the shielding efficiency. These investigations clearly confirmed sufficient shielding efficiency.

For the new expertise of the year 2013 the TÜV Süd expert inspected the plans, calculations and test reports submitted by IPP, inspected the device and hall, got new bore samples produced and analysed, and made extensive calculations of his own. The expert concludes that the concrete of the experimentation hall meets the radiation protection requirements in every respect.

What is the shielding efficiency of the hall gateways?The gateways were made of heavy-duty or radiation-protection concrete such as has already been used in many cases for radiation protection in Germany. As previously with the walls of the experimentation hall, GRS came to a positive assessment of the shielding efficiency of the gateways. The quality of the concrete is confirmed by the expertise of 2013.

Are cracks in the concrete of the hall roof a problem?During construction small cracks appeared when the roof concrete dried. This is not uncommon with concrete structures. The permit authority was informed at the time. These cracks were discussed with the architect and the building companies with respect to their significance for the statics and were classed as insignificant. The significance of these cracks for radiation protection was assessed by IPP. Large safety reserves and completely adequate shielding efficiency were confirmed. The Social Ministry confirmed this in 1998, as also did the 2013 expertise.

What costs are incurred for Wendelstein 7-X?For the entire period till completion in 2014 the costs of the actual fusion device (investments) are approx. 370 million euros. Construction of the building came to 100 million euros, personnel costs to approx. 310 million euros, and operation to approx. 280 million euros. This makes the total cost for all activities of the Greifswald branch of IPP approx. 1,060 million euros, covering the period 1997 to 2014.

What is the present personnel strength at IPP Greifswald?The Greifswald branch has 390 members of staff at present, plus a number of apprentices, work students, undergraduates, trainees and scholarship holders.